Acousto-optic system having phase-shifting reflector

ABSTRACT

A beam positioner for deflecting a beam path, along which a diffracted beam of linearly polarized laser light is propagatable, within a two-dimensional scan field, the beam positioner includes a first acousto-optic deflectors (AOD) to deflect the beam path within a first one-dimensional scan field extending along a first axis of the two-dimensional scan field, a second AOD to deflect the beam path within a second one-dimensional scan field extending along a second axis of the two-dimensional scan field, a phase retarder arranged between the first AOD and the second AOD and within the beam path along which the beam of laser light is propagatable from the first AOD and a mirror arranged between the first AOD and the second AOD and within the beam path along which the beam of laser light is propagatable from the first AOD.

BACKGROUND

Acousto-optic (AO) devices, sometimes referred to as Bragg cells,diffract and shift light using acoustic waves at radio frequency. Thesedevices are often used for Q-switching, signal modulation intelecommunications systems, laser scanning and beam intensity control inmicroscopy systems, frequency shifting, wavelength filtering inspectroscopy systems. Many other applications lend themselves to usingacousto-optic devices. For example, AO deflectors (AODs) can be used inlaser-based materials processing systems.

In a typical AO device, a transducer is attached to an AO medium (alsoreferred to as an “AO cell”), typically a crystal or glass that issuitably transparent to the wavelength of light to be diffracted. An RFsignal (also known as a “drive signal”) is applied to the transducer(e.g., from an RF driver), thereby driving the transducer to vibrate ata certain frequency to create an acoustic wave that propagates in the AOmedium, manifested as periodic regions of expansion and compression inthe AO medium, thereby creating a periodically changing refractive indexwithin the AO medium. The periodically changing refractive indexfunctions like an optical grating that can diffract a beam of laserlight propagating through the AO medium.

Referring to FIG. 1 , an AOD 100 generally includes AO medium 102, atransducer 104 attached to the AO medium 102 (i.e., at a transducer endof the AO medium 102) and, can also include an acoustic absorber 106attached to the AO medium 102 (i.e., at an absorber end of the AO medium102, opposite the transducer end). An RF driver 108 is usuallyelectrically coupled to an input of the transducer 104 to drive the AOD100. The material from which the AO medium 102 is formed is selecteddepending on the wavelength of light in the beam of laser light to bedeflected. The transducer 104 is generally a piezoelectric transducer,and is operative to vibrate in response to an input RF signal (i.e.,drive signal) output by the RF driver 108. The RF driver 108 isoperative to generate the drive signal that is ultimately input to thetransducer 104.

Generally, the transducer 104 is attached to the AO medium 102 such thatvibrations generated by the transducer 104 can create a correspondingacoustic wave (e.g., as indicated by lines 112) that propagates withinthe AO medium 102, from the transducer end toward the acoustic absorber106 along the diffraction axis 110 of the AOD 100. As exemplarilyillustrated in FIG. 1 , when a drive signal (e.g., characterized by afrequency, amplitude, phase, etc.) is applied to the transducer 104, thetransducer 104 vibrates to create an acoustic wave propagating withinthe AO medium 102, thereby generating a periodically changing refractiveindex within the AO medium 102. As is known in the art, the periodicallychanging refractive index functions to diffract a beam of laser light(e.g., propagating along beam path 114) that is incident upon a firstsurface 102 a of the AO medium 102 and propagates through the AO medium102 at the Bragg angle, θB, measured relative to the acoustic wave.

Diffracting the incident beam of laser light produces a diffractionpattern that typically includes zeroth- and first-order diffractionpeaks, and may also include higher-order diffraction peaks (e.g.,second-order, third-order, etc.). As is known in the art, the portion ofthe diffracted beam of laser light in the zeroth-order diffraction peakis referred to as a “zeroth-order” beam, the portion of the diffractedbeam of laser light in the first-order diffraction peak is referred toas a “first-order” beam, and so on. Generally, the zeroth-order beam andother diffracted-order beams (e.g., the first-order beam, etc.)propagate along different beam paths upon exiting the AO medium 102(e.g., through a second surface 102 b of the AO medium 102, opposite thefirst surface 102 a). For example, the zeroth-order beam propagatesalong a zeroth-order beam path, the first-order beam propagates along afirst-order beam path, and so on. The angles between the zeroth- andother diffracted-order beam paths (e.g., the angle, θD, between thezeroth- and first-order beam paths) corresponds to the frequency (orfrequencies) in the drive signal that was applied to diffract the beamof laser light incident upon the AO medium 102.

The amplitude of the applied drive signal can have a non-linear effecton the proportion of the incident beam of laser light that getsdiffracted into the various diffracted-order beams, and an AOD can bedriven to diffract a significant portion of an incident beam of laserlight into the first-order beam, leaving a relatively small portion ofthe incident beam of laser light to remain in other diffracted-orderbeams (e.g., the zeroth-order beam, etc.). Moreover, the frequency ofthe applied drive signal can be rapidly changed to scan first-order beam(e.g., to facilitate processing of different regions of a workpiece).Thus AODs are advantageously incorporated into laser processing systemsfor use within the field of laser-based materials processing, tovariably deflect the first-order beam onto a workpiece during processing(e.g., melting, vaporizing, ablating, marking, cracking, etc.) of theworkpiece.

Laser processing systems typically include one or more beam dumps toprevent laser light propagating along the zeroth-order beam path (andany higher-order beam paths) from reaching the workpiece. Accordingly,within a laser processing system, the first-order beam path exiting theAOD 100 can typically be regarded as the beam path 114 that has beenrotated or deflected (e.g., by angle, θD, also referred to herein as“first-order deflection angle”) within the AOD 100. The axis about whichthe beam path 114 is rotated (also referred to herein as the “rotationaxis”) is orthogonal to the diffraction axis of the AOD 100 and an axisalong which the incident beam of laser light propagates (also referredto herein as the “optical axis”) within the AOD 100 when the AOD 100 isdriven to diffract the incident beam of laser light. The AOD 100 thusdeflects an incident beam path 114 within a plane (also referred toherein as a “plane of deflection”) that contains (or is otherwisegenerally parallel to) the diffraction axis of the AOD 100 and theoptical axis within the AOD 100. The spatial extent across which the AOD100 can deflect the beam path 114 within the plane of deflection isherein referred to as the “scan field” of the AOD 100.

Laser processing systems can incorporate multiple AODs, arranged inseries, to deflect the beam path 114 along two axes. For example, andwith reference to FIG. 2 , a first AOD 200 and a second AOD 202 can beoriented such that their respective diffraction axes (i.e., a firstdiffraction axis 200 a and a second diffraction axis 202 a,respectively) are oriented perpendicular to one another. In thisexample, the first AOD 200 is operative to rotate the beam path 114about a first rotation axis 200 b (e.g., which is orthogonal to thefirst diffraction axis 200 a), thus deflecting the incident beam path114 within a first plane of deflection (i.e., a plane that contains, oris otherwise generally parallel to, the first diffraction axis 200 a andthe optical axis within the first AOD 200), wherein the first plane ofdeflection is orthogonal to the first rotation axis 200 b. Likewise, thesecond AOD 202 is operative to rotate the beam path 114 about a secondrotation axis 202 b (e.g., which is orthogonal to the second diffractionaxis 202 a), thus deflecting the incident beam path 114 within a secondplane of deflection (i.e., a plane that contains, or is otherwisegenerally parallel to, the second diffraction axis 202 a and the opticalaxis within the second AOD 202), wherein the second plane of deflectionis orthogonal to the second rotation axis 202 b. In view of the above,the first and second AODs 200 and 202 can be collectively characterizedas a multi-axis “beam positioner,” and each can be selectively operatedto deflect the beam path 114 within a two-dimensional scan field 204. Aswill be appreciated, the two-dimensional range scan field 204 can beconsidered to be a superposition of two one-dimensional scan fields: afirst, one-dimensional scan field associated with the first AOD 200 anda second, one-dimensional scan field associated with the second AOD 202.

Depending on the type of AODs included in the multi-axis beampositioner, it can be desirable to rotate the plane of polarization oflight (i.e., the plane in which the electric field oscillates) in thefirst-order beam path transmitted by the first AOD 200. Rotating theplane of polarization will be desired if the amount of RF drive powerrequired to diffract significant portion of an incident beam of laserlight into the first-order beam is highly dependent on the polarizationstate of the beam of laser light being deflected. Further, if each AODin the multi-axis beam positioner includes an AO medium 102 formed ofthe same material, and if each AOD uses the same type of acoustic waveto deflect an incident beam of laser light, and if it is desirable tohave the polarization state of light in the first-order beam transmittedby the first AOD 200 be linear and be oriented in a particular directionrelative to the second diffraction axis 202 a, then it would besimilarly desirable to have the polarization state of light in thefirst-order beam transmitted by the second AOD 202 be rotated withrespect to the polarization state of the light in the first-order beamtransmitted by the first AOD 200 just as the orientation of the secondAOD 202 is rotated with respect to an orientation of the first AOD 200.

Conventionally, the polarization rotation is provided by a half-waveplate, and the orientation of polarization after the half-wave platerelative to the incident beam of laser light is a function of theorientation of the half-wave plate relative to the polarizationorientation of the incident beam of laser light. Half-wave plates aretypically manufactured from materials that exhibit sufficientbirefringence, and which are suitably transparent to a particularwavelength (or range of wavelengths) of light to be phase-shifted.Conventional half-wave plates designed to phase-shift light atwavelengths in a range from 9 μm (or thereabout) 11 μm (or thereabout)(e.g., 9.2 μm, 9.5 μm, 10.6 μm, etc.) are undesirably expensive, andtypically are not suitable for high power laser applications such aslaser-based materials processing with a CO2 laser.

SUMMARY

One embodiment broadly characterized herein A beam positioner fordeflecting a beam path, along which a diffracted beam of linearlypolarized laser light is propagatable, within a two-dimensional scanfield, the beam positioner includes: a first acousto-optic deflector(AOD) to diffract the laser light so as to deflect the beam path withina first one-dimensional scan field extending along a first axis of thetwo-dimensional scan field, a second AOD to diffract the laser light soas to deflect the beam path within a second one-dimensional scan fieldextending along a second axis of the two-dimensional scan field, a phaseretarder arranged between the first AOD and the second AOD and withinthe beam path along which the beam of laser light is propagatable fromthe first AOD; and a mirror arranged between the first AOD and thesecond AOD and within the beam path along which the beam of laser lightis propagatable from the first AOD.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an acousto-optical deflector (AOD).

FIG. 2 schematically illustrates an arrangement of AODs in a multi-axisbeam positioner.

FIGS. 3, 4, 5 and 7 are perspective views schematically illustratingmulti-axis beam positioners according to various embodiments.

FIGS. 6 and 8 are graphs illustrating exemplary relationships betweenphase shift and angle of incidence for quarter-wave phase-shiftingreflectors in multi-axis beam positioners according to variousembodiments.

DETAILED DESCRIPTION

Example embodiments are described herein with reference to theaccompanying drawings. Unless otherwise expressly stated, in thedrawings the sizes, positions, etc., of components, features, elements,etc., as well as any distances therebetween, are not necessarily toscale, but are exaggerated for clarity. In the drawings, like numbersrefer to like elements throughout. Thus, the same or similar numbers maybe described with reference to other drawings even if they are neithermentioned nor described in the corresponding drawing. Also, evenelements that are not denoted by reference numbers may be described withreference to other drawings.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. Unlessotherwise defined, all terms (including technical and scientific terms)used herein have the same meaning as commonly understood by one ofordinary skill in the art. As used herein, the singular forms “a,” “an”and “the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It should be recognized that theterms “comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof. Unless otherwise specified,a range of values, when recited, includes both the upper and lowerlimits of the range, as well as any sub-ranges therebetween. Unlessindicated otherwise, terms such as “first,” “second,” etc., are onlyused to distinguish one element from another. For example, one nodecould be termed a “first node” and similarly, another node could betermed a “second node”, or vice versa.

Unless indicated otherwise, the term “about,” “thereabout,”“approximately,” etc., means that amounts, sizes, formulations,parameters, and other quantities and characteristics are not and neednot be exact, but may be approximate and/or larger or smaller, asdesired, reflecting tolerances, conversion factors, rounding off,measurement error and the like, and other factors known to those ofskill in the art. Spatially relative terms, such as “below,” “beneath,”“lower,” “above,” and “upper,” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element or feature, as illustrated in the FIGS. It should berecognized that the spatially relative terms are intended to encompassdifferent orientations in addition to the orientation depicted in theFIGS. For example, if an object in the FIGS. is turned over, elementsdescribed as “below” or “beneath” other elements or features would thenbe oriented “above” the other elements or features. Thus, the exemplaryterm “below” can encompass both an orientation of above and below. Anobject may be otherwise oriented (e.g., rotated 90 degrees or at otherorientations) and the spatially relative descriptors used herein may beinterpreted accordingly.

Embodiments of the present invention can be generally characterized asproviding a multi-axis beam positioner including at least onephase-shifting reflector (also known in the art as a “phase shiftingmirror,” a “phase retarding mirror,” a “reflective phase retarder,”etc.) disposed in the path of the beam of laser light transmitted by anAOD. The beam of laser light transmitted by the AOD can be generallycharacterized as being linearly polarized, and the at least onephase-shifting reflector is configured and oriented so as to rotate theplane of polarization of the beam of laser light transmitted by the AOD.

In one example embodiment, shown in FIG. 3 , a multi-axis beampositioner 300 may include a first AOD 302 (e.g., characterized by afirst diffraction axis 302 a and a first rotation axis 302 b), a secondAOD 304 (e.g., characterized by a second diffraction axis 304 a and asecond rotation axis 304 b), a phase-shifting reflector 306, and anoptical relay system 308 (e.g., comprising a pair of relay lenses 310and 312). Generally, each of the first AOD 302 and the second AOD 304may be provided as discussed above with respect to the AOD 100. Forexample, each of the first AOD 302 and the second AOD 304 may include anAO medium (such as AO medium 102), a transducer (such as transducer 104)attached to the transducer end of the AO medium and, optionally, anacoustic absorber (such as absorber 106) attached to the AO medium at anabsorber end of the AO medium opposite the transducer end.

Although not illustrated, the multi-axis beam positioner 300 may includeone or more RF drivers (e.g., such as RF driver 108) electricallycoupled to an input of a transducer (also not shown) of each of thefirst AOD 302 and the second AOD 304. Accordingly, one or more drivesignals can be applied to each of the first AOD 302 and the second AOD304 by an RF driver. In response to an applied drive signal, the firstAOD 302 is operative to deflect an incident beam of laser light within afirst plane of deflection (i.e., a plane orthogonal to the firstrotation axis 302 b, and containing or otherwise parallel to the firstdiffraction axis 302 a and the optical axis within the first AOD 302).Likewise, in response to an applied drive signal, the second AOD 304 isoperative to deflect an incident beam of laser light within a secondplane of deflection (i.e., a plane orthogonal to the second rotationaxis 304 b, and containing or otherwise parallel to the seconddiffraction axis 304 a and the optical axis within the second AOD 304).

The half-wave phase-shifting reflector 306 is provided as a half-wavephase-shifting reflector (e.g., having a substantially planar reflectorsurface 306 a) configured to effect a 180 degree phase shift between theS and P polarization components of the incident beam of laser light. Theoptical relay system 308 is arranged and configured to relay an image ofthe first AOD 302 onto the second AOD 304. As shown herein, the beampath 114 is graphically-illustrated as a dash-dot line, and theaforementioned components of the multi-axis beam positioner 300 arearranged so as to either diffract (e.g., in the case of the first AOD302 and second AOD 304), refract (e.g., in the case of the optical relaysystem 308) or reflect (e.g., in the case of the half-wavephase-shifting reflector 306) laser light propagating along the beampath 114.

The first AOD 302 and the second AOD 304 are each provided aslongitudinal-mode AODs. Accordingly, the plane of polarization of laserlight incident upon any particular AOD is parallel to (or at leastsubstantially parallel to) the plane of polarization of laser light thatexits that AOD. The multi-axis beam positioner 300 is configured tooperate on linearly polarized laser light and, so, laser lightpropagating along beam path 114 and incident upon the first AOD 302 isprovided so as to be linearly polarized (or at least substantiallylinearly polarized) by any means known in the art, and the first AOD 302is oriented such that the first diffraction axis 302 a is parallel with(or at least substantially parallel with) the plane polarization of thebeam of laser light incident thereto. Likewise, laser light propagatingalong beam path 114 and incident upon the second AOD 304 is linearlypolarized (or at least substantially linearly polarized), and the secondAOD 304 is oriented such that the second diffraction axis 304 a isparallel with (or at least substantially parallel with) the planepolarization of the beam of laser light incident thereto.

The half-wave phase-shifting reflector 306 is arranged and configured torotate the plane of polarization of laser light (i.e., relative to thefirst plane of deflection of the first AOD 302) that is incident uponthe reflector surface 306 a (i.e., after exiting the first AOD 302) by90 degrees. To achieve this, and as will be discussed in greater detailbelow, the half-wave phase-shifting reflector 306 is oriented such thatthe beam of laser light is incident upon the reflector surface 306 a atan angle of incidence of 45 degrees (or thereabout). In addition, thehalf-wave phase-shifting reflector 306 is oriented such that the planeof polarization of the incident beam of laser light is at an angle of 45degrees (or at least substantially 45 degrees) relative to the plane ofincidence/reflection at the reflector surface 306 a.

During operation, the frequency contained in any drive signal to beapplied to the first AOD 302 may be within an intended range offrequencies which, when applied to the first AOD 302, generate afirst-order diffracted beam propagating exiting the first AOD 302 at afirst-order deflection angle, θD, that is within a range of first-orderdeflection angles (also referred to herein as the “first-orderdeflection angle range”). The intended frequency range can beconceptually considered as a frequency band spanning a range offrequencies that is bounded by a lower frequency an upper frequency.

In one embodiment, the orientation of the half-wave phase-shiftingreflector 306 is fixed relative to the first AOD 302. Thus duringoperation of the first AOD 302, the first-order beam path 114 exitingthe first AOD 302 may be incident upon the reflector surface 306 a atone of many possible angles of incidence (i.e., depending upon thefrequency contained in the drive signal applied to the first AOD 302during operation of the first AOD 302). In one embodiment, the half-wavephase-shifting reflector 306 is oriented such that the first-order beampath 114 exiting the first AOD 302 is incident on the reflector surface306 a at an angle of incidence of 45 degrees (or thereabout, orotherwise at an angle of incidence of at least substantially 45 degrees)when the frequency of the drive signal applied to the first AOD 302 isequal to a reference frequency within the frequency band of the intendedfrequency range. The frequency band may be equal to 2 MHz, 5 MHz, 10MHz, 15 MHz, 20 MHz, 25 MHz, 30 MHz, etc., or between any of thesevalues, and the lower frequency of the frequency band may be equal to 25MHz, 30 MHz, 35 MHz, 40 MHz, 45 MHz, 50 MHz, 55 MHz, 60 MHz, etc., orbetween any of these values. Accordingly, the reference frequency may beany frequency in a range from 26 MHz (or thereabout) to 89 MHz (orthereabout). In one embodiment, the reference frequency may be 30 MHz,40 MHz, 50 MHz, 60 MHz, 70 MHz, 80 MHz, etc., or between any of thesevalues. Generally, the reference frequency is located at or near themiddle of the frequency band of the intended frequency range. In oneembodiment, the reference frequency is near the middle of the frequencyband of the intended frequency range when the reference frequency iswithin 15%, 10%, 5%, 2%, 1%, 0.5%, 0.25%, 0.1%, etc., or between any ofthese values, of the middle frequency in the frequency band.

In another embodiment, the orientation of the half-wave phase-shiftingreflector 306 relative to the first AOD 302 can be variable. Forexample, the half-wave phase-shifting reflector 306 may be rotated toensure that the first-order beam path 114 exiting the first AOD 302 isincident on the reflector surface 306 a at an angle of incidence of 45degrees (or thereabout, or otherwise at an angle of incidence of atleast substantially 45 degrees) when the frequency of the drive signalapplied to the first AOD 302 is within a sub-range of intendedfrequencies. The sub-range of intended frequencies can be considered asa frequency band spanning a sub-range of frequencies (which may be equalto or less than the intended frequency range) bounded by a lowerfrequency an upper frequency. To facilitate rapid adjustments in theorientation of the half-wave phase-shifting reflector 306 relative tothe first AOD 302, the half-wave phase-shifting reflector 306 may bemounted to a stage that is actuated by a voice coil actuator, apiezoelectric-positioner, micro-electro-mechanical system (MEMS)positioner, or the like or any combination thereof), or the half-wavephase-shifting reflector 306 may be provided as a deformable mirror, orthe like or any combination thereof.

As illustrated, the phase-shifting reflector 306 is disposed in the beampath 114 between the first AOD 302 and the optical relay system 308. Inanother embodiment, however, the phase-shifting reflector 306 can bedisposed in the beam path 114 between the pair of relay lenses 310 and312 of the optical relay system 308. In yet another embodiment, thephase-shifting reflector 306 can be disposed in the beam path 114between the optical relay system 308 and the second AOD 304.

When oriented and configured as described above, the half-wavephase-shifting reflector 306 rotates the plane of polarization of anincident beam of laser light (i.e., about the optical axis along whichthe beam of laser light propagates) by 90 degrees with respect to thefirst plane of deflection of the first AOD 302. Further, and asexemplarily illustrated in FIG. 3 , the half-wave phase-shiftingreflector 306 skews an orientation of the beam path 114 in a manner thatcan make it difficult to assemble the components of the multi-axis beampositioner 300 into a relatively compact package. To facilitate amore-compact assembly of the multi-axis beam positioner 300, a mirrorconfigured to impart zero (or at least substantially zero) phase shift(also referred to herein as a “zero phase-shift reflector”) can beprovided to fold the beam path 114 in any suitable or desired manner toprovide a more-compact multi-axis beam positioner.

For example, and with reference to FIG. 4 , a zero phase-shift reflector402 (e.g., having a substantially planar reflector surface 402 a) may beinserted into the beam path 114 between the half-wave phase-shiftingreflector 306 and lens 310 of the multi-axis beam positioner 300(thereby yielding multi-axis beam positioner 400). As exemplarily shownin FIG. 4 , the zero phase-shift reflector 402 can be oriented such thatthe beam of laser light reflected by the zero phase-shift reflector 402propagates along a direction that is generally opposite to the directionin which the beam of laser light propagates on incidence to thehalf-wave phase-shifting reflector 306 and such that the first plane ofdeflection of the first AOD 302, as reflected from the zero phase-shiftreflector 402, is rotated by 90 degrees relative to the orientation ofthe first plane of deflection as incident upon the reflector surface 306a. Because the zero phase-shift reflector 402 does not impart any (orany substantial) phase shift, the plane of polarization of beam of laserlight reflected at the reflector surface 402 a does not change (orchanges a negligible amount) relative to the first plane of deflectionof the first AOD 302. As a result, the direction of polarization of thelinearly-polarized laser light ultimately delivered to the second AOD304 will be parallel to (or at least substantially parallel to) thedirection of polarization of the linearly-polarized laser light outputfrom the first AOD 302. Thus, as illustrated in FIG. 4 , the seconddiffraction axis 304 a of the second AOD 304 can be parallel to (or atleast substantially parallel to) the first diffraction axis 302 a of thefirst AOD 302. Further, the first plane of deflection of the first AOD302, as it is projected onto the second AOD 304 (e.g., from the zerophase-shift reflector 402 via the optical relay system 308), will beperpendicular to (or at least substantially perpendicular to) the secondplane of deflection of the second AOD 304. Thus the scan fieldassociated with the first AOD 302 (a one-dimensional scan field), as itis projected onto the second AOD 304, will be perpendicular to (or atleast substantially perpendicular to) the scan field associated with thesecond AOD 304 (also a one-dimensional scan field), and the multi-axisbeam positioner 400 can be considered as having a two-dimensional scanfield characterized by the superposition of the two, one-dimensionalscan fields associated with the first AOD 302 and the second AOD 304.

Typically, the amount of phase shift (also known as “phase retardation”)that the half-wave phase-shifting reflector 306 can impart to anincident beam of laser light propagating along the beam path 114 willchange as the angle of incidence of the beam path 114 at the reflectorsurface 306 a changes (e.g., as a result of changing the drive frequencyof the first AOD 302). This change in phase shift would result in adeviation of the polarization state of the beam that is incident uponthe second AOD 304 such that it is no longer linearly polarized in thedesired axis, but rather is elliptically polarized. To eliminate orotherwise reduce the effect of a variable angle of incidence of the beampath 114 at the reflector surface 306 a, the half-wave phase-shiftingreflector 306 may be replaced with a pair of quarter-wave phase-shiftingreflectors. For example, and as shown in FIG. 5 , a multi-axis beampositioner 500 may be provided as similarly described with respect tothe multi-axis beam positioner 300 shown in FIG. 3 , but the half-wavephase-shifting reflector 306 is replaced by a pair of quarter-wavephase-shifting reflectors (i.e., a first phase-shifting reflector 502having a substantially planar reflector surface 502 a and a secondphase-shifting reflector 504 having a substantially planar reflectorsurface 504 a) arranged within the beam path 114.

As exemplarily shown in FIG. 5 , the first quarter-wave phase-shiftingreflector 502 is oriented such that a beam of laser light propagatingalong beam path 114 will be incident upon the reflector surface 502 a atan angle of incidence (also referred to as a “first angle of incidence”)of 45 degrees (or thereabout, or otherwise at a first angle of incidenceof at least substantially 45 degrees) when the frequency of the drivesignal applied to the first AOD 302 is equal to the aforementionedreference frequency of the intended frequency range. The firstquarter-wave phase-shifting reflector 502 is further oriented to ensurethat the light reflected from the reflector surface 502 a contains equal(or at least substantially equal) amounts of S and P polarizationcomponents (i.e., so that the light reflected by the reflector surface502 a is circularly-polarized, or at least roughly circularly-polarized)when the frequency of the drive signal applied to the first AOD 302 isequal to the aforementioned reference frequency of the intendedfrequency range. When oriented as described above, the firstquarter-wave phase-shifting reflector 502 is thus configured to effect aphase shift between the S and P polarized components of an incident beamof laser light by 90 degrees (or thereabout).

The second quarter-wave phase-shifting reflector 504 is oriented suchthat the reflector surface 504 a is perpendicular to (or at leastsubstantially perpendicular to) the reflector surface 502 a of the firstquarter-wave phase-shifting reflector 502. Thus, the surface normal ofthe reflector surface 504 a of the second quarter-wave phase-shiftingreflector 504 is perpendicular to (or at least substantiallyperpendicular to) the surface normal of the reflector surface 502 a ofthe first quarter-wave phase-shifting reflector 502. When oriented asdescribed above, the second quarter-wave phase-shifting reflector 504 isconfigured to effect a phase shift between the S and P polarizedcomponents of an incident beam of laser light by 90 degrees (orthereabout). Accordingly, circularly-polarized (or at least roughlycircularly-polarized) light that is incident upon the reflector surface504 a will be reflected as linearly-polarized (or at least substantiallylinearly-polarized) light.

To facilitate a combined phase shift of 180 degrees (or thereabout) fromthe pair of quarter-wave phase-shifting reflectors 502 and 504 in themulti-axis beam positioner 500, the first quarter-wave phase-shiftingreflector 502 is provided to have the same (or substantially the same)phase-shifting characteristics (which may be at least substantiallylinear) as the second quarter-wave phase-shifting reflector 504 over thesame range of angles of incidence. FIG. 6 is a chart illustratingexemplary phase-shifting characteristics that each of the firstquarter-wave phase-shifting reflector 502 and the second quarter-wavephase-shifting reflector 504 may have over the same range of angles ofincidence. The amount of phase shift imparted by either the firstquarter-wave phase-shifting reflector 502 or the second quarter-wavephase-shifting reflector 504 at any particular angle of incidence hasthe potential to vary (e.g., as indicated by the error bars) dependingupon one or more factors such as the material from which the reflectorsurface of the quarter-wave phase-shifting reflector is made, thetemperature of the reflector surface, the presence, magnitude andorientation of any mechanical strain at the reflector surface, or thelike or any combination thereof.

When the first-order beam path 114 exiting the first AOD 302 is incidenton the reflector surface 502 a at a first angle of incidence of 45degrees, the first quarter-wave phase-shifting reflector 502 will imparta 90 degree phase shift to the linearly polarized laser light incidentthereupon, and reflect a beam of laser light that is at leastsubstantially circularly polarized (e.g., with at least substantiallyequal amounts of S and P polarization components). However, and asexemplarily shown in FIG. 6 , when the first angle of incidence deviatesaway from 45 degrees, the phase shift imparted by the first quarter-wavephase-shifting reflector 502 correspondingly deviates away from 90degrees, resulting in a reflected beam of laser light having apolarization that becomes increasingly elliptical.

For example, as the first angle of incidence increases above 45 degrees,the first quarter-wave phase-shifting reflector 502 will produce a phaseshift greater than 90 degrees (i.e., an “overshift”). As the angle ofincidence decreases below 45 degrees, the first quarter-wavephase-shifting reflector 502 will produce a phase shift less than 90degrees (i.e., an “undershift”). However, when the second quarter-wavephase-shifting reflector 504 is oriented as described above, a secondangle of incidence (i.e., the angle of incidence of the beam of laserlight incident upon the reflector surface 504 a) is a complement to thefirst angle of incidence. That is, the sum of the first and secondangles of incidence is 90 degrees. Accordingly, an overshift produced bythe first quarter-wave phase-shifting reflector 502 is compensated forby an equal (or approximately or at least substantially equal) butopposite undershift produced by the second quarter-wave phase-shiftingreflector 504. Likewise, an undershift produced by the firstquarter-wave phase-shifting reflector 502 is compensated for by an equal(or approximately or at least substantially equal) but oppositeovershift produced by the second quarter-wave phase-shifting reflector504. The net result is that the first quarter-wave phase-shiftingreflector 502 and second quarter-wave phase-shifting reflector 504,together, can effect a combined phase shift of 180 degrees (orthereabout) between the S and P components in the beam of laser lightthat is incident upon the first quarter-wave phase-shifting reflector502 over a range of angles of incidence.

When oriented and configured as described above, the first quarter-wavephase-shifting reflector 502 and second quarter-wave phase-shiftingreflector 504 of the multi-axis beam positioner 500 act together torotate the plane of polarization of the beam of laser light output fromthe first AOD 302 (e.g., by 90 degrees, or thereabout) relative to thefirst plane of deflection of the first AOD 302, and to also rotate thefirst plane of deflection of the first AOD 302 (e.g., by 90 degrees, orthereabout) relative to the orientation of the first plane of deflectionas incident upon the reflector surface 502 a, and the multi-axis beampositioner 500 can thus be considered as having a two-dimensional scanfield characterized by the superposition of the two, one-dimensionalscan fields associated with the first AOD 302 and the second AOD 304.

As illustrated in FIG. 5 , the first quarter-wave phase-shiftingreflector 502 and second quarter-wave phase-shifting reflector 504 aredisposed between the first AOD 302 and the optical relay system 308. Inanother embodiment, however, the first quarter-wave phase-shiftingreflector 502 and second quarter-wave phase-shifting reflector 504 canbe disposed between the pair of relay lenses 310 and 312 of the opticalrelay system. In yet another embodiment, the first quarter-wavephase-shifting reflector 502 and second quarter-wave phase-shiftingreflector 504 can be disposed between the optical relay system 308 andthe second AOD 302. In yet another embodiment, the first quarter-wavephase-shifting reflector 502 can be disposed between any pair of thecomponents in the multi-axis beam positioner 500 and the secondquarter-wave phase-shifting reflector 504 can be disposed opticallydownstream of the first quarter-wave phase-shifting reflector 502,between another pair of the components in the multi-axis beam positioner500.

In another embodiment, the multi-axis beam positioner 500 can bemodified such that the reflector surface 504 a of the secondquarter-wave phase-shifting reflector 504 is parallel to (or at leastsubstantially parallel to) the reflector surface 502 a of the firstquarter-wave phase-shifting reflector 502 (thereby yielding themulti-axis beam positioner 700 shown in FIG. 7 ). In addition to thereflector surfaces 502 a and 504 a being parallel to (or at leastsubstantially parallel to) one another, the plane of reflection of thesecond quarter-wave phase-shifting reflector 504 is the same (or atleast substantially coplanar as) the plane of reflection of the firstquarter-wave phase-shifting reflector 502. Thus, the surface normal ofthe reflector surface 504 a of the second quarter-wave phase-shiftingreflector 504 would be parallel to (or at least substantially parallelto) the surface normal of the reflector surface 502 a of the firstquarter-wave phase-shifting reflector 502.

To facilitate a combined phase shift of 180 degrees (or about 180degrees) from the pair of quarter-wave phase-shifting reflectors 502 and504 in the multi-axis beam positioner 700, the first quarter-wavephase-shifting reflector 502 is provided to have correspondinglydifferent phase-shifting characteristics (which may be at leastsubstantially linear) as the second quarter-wave phase-shiftingreflector 504 over the same range of angles of incidence. Specifically,one of the first quarter-wave phase-shifting reflector 502 or the secondquarter-wave phase-shifting reflector 504 is configured to produce anundershift of phase between S and P polarization components at a givenangle of incidence (by any suitable or beneficial means known in theart) while the other of the first quarter-wave phase-shifting reflector502 or the second quarter-wave phase-shifting reflector 504 isconfigured to produce an overshift of phase between S and Ppolarizations at the same given angle of incidence. FIG. 8 is a chartillustrating exemplary phase-shifting characteristics that one of thefirst quarter-wave phase-shifting reflector 502 or the secondquarter-wave phase-shifting reflector 504 may have over a range ofangles of incidence. For example, the first quarter-wave phase-shiftingreflector 502 may have the phase-shifting characteristics shown in FIG.6 while the second quarter-wave phase-shifting reflector 504 may havethe phase-shifting characteristics shown in FIG. 8 , or vice-versa.

As shown in FIGS. 6 and 8 , the combined magnitude of the undershift andovershift at a common angle of incidence will be 180 degrees (orthereabout). For example, at an angle of incidence of 45 degrees, bothof the quarter-wave phase-shifting reflectors produce a 90 degree phaseshift. At an angle of incidence of 43 degrees, one of the quarter-wavephase-shifting reflectors produces an 83 degree phase shift (see FIG. 6) while the other of the quarter-wave phase-shifting reflectors producesa 97 degree phase shift (see FIG. 8 ). At an angle of incidence of 46degrees, one of the quarter-wave phase-shifting reflectors produces aphase shift of 94 degrees (see FIG. 6 ) while the other of thequarter-wave phase-shifting reflectors produces a phase shift of 86degrees (see FIG. 8 ).

When oriented and configured as described above, the first quarter-wavephase-shifting reflector 502 and second quarter-wave phase-shiftingreflector 504 of the multi-axis beam positioner 700 act together torotate the plane of polarization of the beam of laser light output fromthe first AOD 302 (e.g., by 90 degrees, or thereabout) relative to thefirst plane of deflection of the first AOD 302. Unlike the embodimentdiscussed above with respect to the multi-axis beam positioner 500,however, the pair of quarter-wave phase-shifting reflectors in themulti-axis beam positioner 700 do not rotate the first plane ofdeflection of the first AOD 302 relative to the plane of polarization oflaser light incident upon the reflector surface 502 a of the firstquarter-wave phase-shifting reflector 502. The pair of quarter-wavephase-shifting reflectors can also be considered to redirect the beampath 114 such that light reflected from the reflector surface 504 apropagates in a direction that is generally the same as the direction inwhich the beam of laser light was incident upon the reflector surface502 a of the first quarter-wave phase-shifting reflector 502. As aresult, the direction of polarization of the linearly-polarized laserlight ultimately delivered to the second AOD 304 in the multi-axis beampositioner 700 will be perpendicular to (or at least substantiallyperpendicular to) the direction of polarization of thelinearly-polarized laser light output from the first AOD 302. Thus, asillustrated in FIG. 7 , the second diffraction axis 304 a of the secondAOD 304 can be perpendicular to (or at least substantially perpendicularto) the first diffraction axis 302 a of the first AOD 302. Further, thefirst plane of deflection of the first AOD 302, as it is projected ontothe second AOD 304 (e.g., from the second quarter-wave phase-shiftingreflector 504 via the optical relay system 308), will be perpendicularto (or at least substantially perpendicular to) the second plane ofdeflection of the second AOD 304. Thus the scan field associated withthe first AOD 302 (a one-dimensional scan field), as it is projectedonto the second AOD 304, will be perpendicular to (or at leastsubstantially perpendicular to) the scan field associated with thesecond AOD 304 (also a one-dimensional scan field), and the multi-axisbeam positioner 700 can be considered as having a two-dimensional scanfield characterized by the superposition of the two, one-dimensionalscan fields associated with the first AOD 302 and the second AOD 304.

From the discussion above it is assumed that, in the multi-axis beampositioner 700, the orientation of the first quarter-wave phase-shiftingreflector 502 is fixed relative to the first AOD 302 and that theorientation of the second quarter-wave phase-shifting reflector 504 isfixed relative to the first quarter-wave phase-shifting reflector 502.In this embodiment, and unlike the embodiment discussed with respect toFIG. 5 , the second quarter-wave phase-shifting reflector 504 does notcompensate for overshift or undershift if the two phase-shiftingreflectors impart the same phase shift between S and P polarizationcomponents at an angle of incidence other than an angle of incidencethat imparts a 90 degree phase shift between the S and P polarizationcomponents. However, in other embodiments, the orientation of the firstquarter-wave phase-shifting reflector 502 relative to the first AOD 302can be variable, the orientation of the second quarter-wavephase-shifting reflector 504 relative to the first AOD 302 can bevariable, the orientation of the first quarter-wave phase-shiftingreflector 502 relative to the second quarter-wave phase-shiftingreflector 504 can be variable, the orientation of the secondquarter-wave phase-shifting reflector 504 relative to the firstquarter-wave phase-shifting reflector 502 can be variable, or the likeor any combination thereof. For example, the first quarter-wavephase-shifting reflector 502 may be rotated (e.g., independently of, orin unison with, the second quarter-wave phase-shifting reflector 504) toensure that the first-order beam path 114 exiting the first AOD 302 isincident on the reflector surface 502 a at an angle of incidence of 45degrees (or at least substantially 45 degrees) when the frequency of thedrive signal applied to the first AOD 302 is within the aforementionedsub-range of intended frequencies. In another example, the secondquarter-wave phase-shifting reflector 504 may be rotated relative to thefirst quarter-wave phase-shifting reflector 502 to compensate for anyovershift or undershift produced by the first quarter-wavephase-shifting reflector 502 when the frequency of the drive signalapplied to the first AOD 302 is within the aforementioned sub-range ofintended frequencies. To facilitate rapid adjustments in the orientationof any of the quarter-wave phase-shifting reflectors, one or both of thequarter-wave phase-shifting reflectors may be mounted to a stage that isactuated by a voice coil actuator, a piezoelectric-positioner,micro-electro-mechanical system (MEMS) positioner, or the like or anycombination thereof), or one or both of the quarter-wave phase-shiftingreflectors may be provided as a deformable mirror, or the like or anycombination thereof.

In one embodiment, the material from which the AO medium 102 of thefirst and second AODs 302 and 304 is formed can be a material such asgermanium (Ge), which is typically selected to deflect light having awavelength in a range from 2 μm to 20 μm. Accordingly the beam of laserlight propagating along the beam path 114 can have a wavelength in arange from 2 μm to 20 μm and, in one embodiment, the wavelength is in arange from 9 μm to 11 μm. Exemplary wavelengths can include 9.4 μm, 9.6μm, 10.6 μm, etc., or thereabout or between any of these values. Such abeam of laser light can be generated from any suitable laser source(e.g., a high-power CO2 laser, capable of outputting a laser beam at anaverage power in a range from 20 W (or thereabout) to 20 kW (orthereabout), as is known in the art). The material from which any of theaforementioned phase-shifting reflectors can be formed can include amaterial such as silicon, copper, molybdenum, gold, or the like or anycombination thereof, and as is known in the art, is typically selecteddepending on the wavelength of light in the beam of laser light to bedeflected. For example, the AO cells of the first and second AODs 302and 304 may be formed of germanium (Ge) and any phase-shifting reflectorof any of the multi-axis beam positioners 300, 400, 500 or 700 may beformed of a material such as silicon or copper, and may optionallyinclude one more coatings as is well known in the art.

In the embodiments discussed above, the multi-axis beam positioners 300,400, 500 and 700 are provided as multi-axis beam positioners with twoAODs (i.e., the first and second AODs 200 and 202). In otherembodiments, the beam positioner may include a single AOD, or more thantwo AODs. In an embodiment in which the beam positioner includes asingle AOD, the beam positioner may include at least one phase-shiftingreflector (e.g., at least one half-wave phase-shifting reflector, atleast one quarter-wave phase-shifting reflector, or the like or anycombination thereof) arranged at the optical output of the AOD. In anembodiment in which the beam positioner includes more than two AODs, thebeam positioner may or may not include at least one phase-shiftingreflector (e.g., as described above with respect to any of FIG. 3, 4, 5or 7 ) arranged at the optical output of any AOD from which a beam pathis fed into another AOD.

In the embodiments discussed above, the beam positioner is described asincluding, as beam deflecting devices, one or more AODs. It should berecognized that the beam positioner may additionally include one or moreother beam deflecting devices (e.g., arranged so as to deflect any beamof light transmitted by any of the AODs described above). In such acase, any of such other beam deflecting devices may include anelectro-optic deflector (EOD), a fast-steering mirror (FSM) elementactuated by a piezoelectric actuator, electrostrictive actuator,voice-coil actuator, etc., a galvanometer mirror, a rotating polygonmirror scanner, etc., or the like or any combination thereof.

The foregoing is illustrative of embodiments and examples of theinvention, and is not to be construed as limiting thereof. Although afew specific embodiments and examples have been described with referenceto the drawings, those skilled in the art will readily appreciate thatmany modifications to the disclosed embodiments and examples, as well asother embodiments, are possible without materially departing from thenovel teachings and advantages of the invention.

For example, although the embodiments presented above have discussed theuse of either a half-wave phase-shifting reflector or a pair ofquarter-wave phase-shifting reflectors to effect rotation of the planeof polarization of light output from the first AOD 302, it will beappreciated that any other type of phase-shifting reflector, orcombination of phase-shifting reflectors may be used (with or withoutcooperative of one or more zero phase-shifting reflectors), providedthat such reflectors are configured and oriented to impart a 180 degree(or thereabout) phase shift between the S and P components of polarizedlight in the beam of laser light propagating along beam path 114 so asto rotate the plane of polarization of light output from the first AOD302 by 90 degrees (or thereabout) relative to the first plane ofdeflection of the first AOD 302.

Further, although the discussion above regarding the material from whichthe AO medium 102 of the first and second AODs 200 and 202 is formed hasbeen limited to germanium, it will be appreciated that the material fromwhich the AO medium 102 of any of the first and second AODs 200 and 202can be any other suitable material such as gallium arsenide (GaAs),wulfenite (PbMoO4), tellurium dioxide (TeO2), crystalline quartz, glassySiO2, arsenic trisulfide (As2S3), LiNbO3, or the like, and as is knownin the art, is typically selected depending on the wavelength of lightin the beam of laser light to be deflected. Thus, the materials fromwhich the aforementioned phase-shifting reflectors can be formed willalso depend, as is known in the art, upon the wavelength of light in thebeam of laser light to be reflected. Exemplary materials from which anyphase-shifting reflector can be formed can include a material such asglass, fused silica, crystal quartz, silicon, copper, molybdenum, gold,silicon carbide, aluminum, or the like or any combination thereof.

Further, although the embodiments presented above have discussed the useand arrangement of AODs having a diffraction axis that is parallel to(or at least substantially parallel to) the plane of polarization of thebeam of laser light incident thereto, the principles discussed hereinmay be applied to other embodiments involving the use of AODs having adiffraction axis that is perpendicular to (or at least substantiallyperpendicular to) the plane of polarization of the beam of laser lightincident thereto. For example, each of the first AOD 302 and the secondAOD 304 (i.e., in any of the multi-axis beam positioners 300, 400, 500or 700) can be provided with an AO cell formed of a material such ascrystalline quartz, and be oriented such that the diffraction axis ofeach of these AODs is perpendicular to (or at least substantiallyperpendicular to) the plane of polarization of the beam of laser lightincident to each AOD as the beam of laser light propagates along beampath 114. In this example, the beam of laser light has a wavelength inthe ultraviolet, visible or other infrared ranges of the electromagneticspectrum, and is linearly polarized.

Further, although the embodiments presented above have described themulti-axis beam positioners 300, 400, 500 or 700 as including an opticalrelay system 308, it will be appreciated that the optical relay system308 may be omitted.

Further, although embodiments have been discussed above in whichphase-shifting reflectors are variously used impart a phase shift to thebeam of laser light output from the first AOD 302, it will beappreciated that one or more transmissive phase-shifting plates may alsobe used (e.g., in addition to, or as an alternative to, any of thephase-shifting reflectors discussed above with respect to any of FIG. 3,4, 5 or 7 ). Generally, the transmissive phase-shifting plate is atleast substantially transparent to the wavelength of the beam of laserlight that will propagate along the beam path 114. For example, atransmissive phase-shifting plate, such as a structured-diamondhalf-wave plate, may be inserted into the beam path 114 to impart a 180phase shift to the beam of laser light output by the first AOD 302 whenthe beam of laser light propagating along the beam path 114 has awavelength in a range from 9 μm to 11 μm (e.g., 9.4 μm, 9.6 μm, 10.6 μm,etc., or thereabout or between any of these values).

Accordingly, all such modifications are intended to be included withinthe scope of the invention as defined in the claims. For example,skilled persons will appreciate that the subject matter of any sentence,paragraph, example or embodiment can be combined with subject matter ofsome or all of the other sentences, paragraphs, examples or embodiments,except where such combinations are mutually exclusive. The scope of thepresent invention should, therefore, be determined by the followingclaims, with equivalents of the claims to be included therein.

What is claimed is:
 1. A beam positioner for deflecting a beam path,along which a diffracted beam of linearly polarized laser light ispropagatable, within a two-dimensional scan field, the beam positionercomprising: a first acousto-optic deflector (AOD) operative to diffractthe laser light so as to deflect the beam path within a firstone-dimensional scan field extending along a first axis of thetwo-dimensional scan field; a second AOD operative to diffract the laserlight so as to deflect the beam path within a second one-dimensionalscan field extending along a second axis of the two-dimensional scanfield; a phase retarder arranged between the first AOD and the secondAOD and within the beam path along which the beam of laser light ispropagatable from the first AOD; and a mirror arranged between the firstAOD and the second AOD and within the beam path along which the beam oflaser light is propagatable from the first AOD.
 2. The beam positionerof claim 1, further comprising an optical relay system arranged betweenthe first AOD and the second AOD such that the first one-dimensionalscan field is projectable onto the second AOD via the optical relaysystem.
 3. The beam positioner of claim 2, wherein the phase retarder isarranged between the first AOD and the optical relay system.
 4. The beampositioner of claim 2, wherein the phase retarder is arranged betweenthe second AOD and the optical relay system.
 5. The beam positioner ofclaim 2, wherein the optical relay system includes a pair of lenses andwherein the phase retarder is arranged between the lenses of the pair oflenses.
 6. The beam positioner of claim 2, wherein the first AOD, secondAOD, phase retarder and mirror are arranged such that the beam of laserlight is propagatable through optical relay system in a direction thatis at least generally opposite to a direction in which the beam of laserlight is incident upon the first AOD.
 7. The beam positioner of claim 2,wherein the first AOD, second AOD, phase retarder and mirror arearranged such that the beam of laser light is propagatable throughoptical relay system in a direction that is at least generally the sameas a direction in which the beam of laser light is incident upon thefirst AOD.
 8. The beam positioner of claim 1, wherein the phase retarderis a transmissive phase retarder.
 9. The beam positioner of claim 1,wherein the phase retarder is a reflective phase retarder.
 10. The beampositioner of claim 9, wherein the reflective phase retarder is ahalf-wave reflective phase retarder.
 11. The beam positioner of claim 9,wherein the reflective phase retarder is a half-wave reflective phaseretarder.
 12. The beam positioner of claim 1, wherein the mirror is azero phase-shift mirror.
 13. The beam positioner of claim 1, wherein themirror is a reflective phase-retarder configured to impart a phase shiftbetween S and P components of the linearly polarized beam of laser lightincident thereto.
 14. The beam positioner of claim 13, wherein thereflective phase-retarder is formed of a material including at least onematerial selected from the group consisting of silicon and copper. 15.The beam positioner of claim 1, wherein the first AOD includes an AOcell formed of a material including germanium.
 16. The beam positionerof claim 1, wherein the first AOD includes an AO cell formed of amaterial including quartz.
 17. The beam positioner of claim 1, furthercomprising at least one galvanometer mirror arranged within the beampath, wherein the at least one phase-shifting reflector is arrangedbetween the first AOD and the galvanometer mirror.